In processing steps wherein the workpiece is subject to a high intensity radiation flux, heat developed in the workpiece may become a limiting factor for the process. In particular, for ion implantation of semiconductor materials, an upper limit on the workpiece temperature is recognized for several reasons. Where the wafer is coated with a resist layer as part of a lithographic process, deterioration or alteration of that layer will occur for temperatures elevated much in excess of 100.degree. C. Wafers subject to prolonged irradiation may also experience undesirable diffusion of previously formed regions of distinct incremental properties within the semiconductor or a premature annealing of previously bombarded regions may occur.
It is therefore a matter of importance to provide for the removal from the semiconductor wafer of heat developed therein consequent to ion implantation processes or like irradiation.
It is known in the prior art to provide active cooling for semiconductor wafers during ion implantation by clamping such wafers to a convexly curved platen which includes a coating of a pliable thermally conductive material on the surface of the platen. A clamping ring cooperating with the platen is arranged to firmly press a semiconductor wafer against the compliant surface of the convexly curved platen to facilitate a thermal energy transfer from the wafer to active cooling means provided within the platen. Such a system is described in U.S. Pat. No. 4,282,924 commonly assigned with the present invention.
The aforementioned art relies upon a conductive mechanism for thermal transport. Thermal energy is developed in proximity to the outer surface of the wafer from the kinetic energy of the incident beam which is absorbed by the wafer. There is, therefore, a first component of thermal conductivity implicit in the thermal conductance properties of the wafer material because of the necessity of heat transport through the wafer material. (It will be assumed for simplicity that the thermal path is through the thickness of the wafer.) Similarly, the platen exhibits thermal conductance properties characteristic of the material comprising the platen in effecting heat transfer between the surface of the platen in contact with the wafer, thence through the interior regions of the platen wherein cooling channels are disposed for circulating cooling fluids. In the intermediate contact region between the wafer and the platen there is a distinct contribution to thermal transfer properties. The thermal conductance in this region is nearly proportional to the actual contact area between the wafer and platen and inversely proportional to the mean thermal conductivity of the two materials. On the microscopic scale, the surfaces are quite nonplanar and of irregular orientation; on the basis of certain assumptions of the hardness of materials and surface topography distributions of the respective contact surfaces, the contact area is calculable for microscopic measurement and is, in fact, but a small fraction of the macroscopic area. The theory of conductive heat transport between solid bodies in a vacuum is developed by Cooper, Mikic and Yovanovich, Int. J. Heat and Mass Transfer, Vol. 12, pp. 279-300 (1969) where it is shown that the contact thermal conductance depends upon the conductances and the actual contact area which in turn depends upon the surface density of deviations from planarity of the meeting surfaces and the elastic or plastic compliance of the materials. Irregularities may, by impressed forces bearing thereon, be deformed to initially contact, or to more nearly conform with one another, e.g., yielding a greater contact area. The desirable effect of greater contact area is limited by the maximum stress which can be sustained by the wafer. In U.S. Pat. No. 4,282,924 the platen is in fact a composite of a high heat capacity metal body, of unspecified convex curvature, to which is bonded a thermally conductive compliant outer layer for contacting the wafer. Thus, there is provided a surface layer which deforms to accommodate some portion of the small scale irregularities of the wafer. There is, in this example, a further geometrical attribute in the length of the thermal path through the compliant material, which length is proportionately shortened as the contact pressure increases. This effect appears in first order but is quite small for the small deformations usually encountered.
The effect of variables which influence the thermal impedance of the interface was recognized as providing a selectably controllable thermal impedance where it is desired to sustain a preselected wafer temperature relative to the heat sink. U.S. Pat. No. 4,453,080, commonly assigned, discusses an example of this attribute.
It is therefore an object of the present invention to provide an apparatus for improving the effective cooling of semiconductor wafers during ion implantation and especially with respect to the uniformity of such cooling.